Ultrasonic diagnostic imaging systems may be used in conjunction with surgical and other invasive procedures to assist in the accurate placement of medical instruments such as needles, trocars, and other medical devices inside the body. For example, U.S. Pat. No. 4,697,595 issued to Breyer et al describes a cardiac catheter and a pacing lead carrying an ultrasonic transducer. When this devices is used in conjunction with an ultrasonic imaging system, the location of the catheter or leads inside the body can be represented in the ultrasonic image by the reception or transmission of ultrasonic signals between the imaging transducer and the transducer mounted on the invasive device. This technique of incorporating ultrasonic transducers into invasive medical instruments has also been applied to needle-type devices, as illustrated in U.S. Pat. No. 3,556,079 issued to Omizo and U.S. Pat. No. 4,249,539 issued to Vilkomerson et al.
In the Omizo patent, Doppler interrogating waves are directed forward from the tip of a biopsy needle. As the needle penetrates the body, backscatter waves from moving fluids within a vessel or organ are received and a conventional Doppler beat frequency is detected. The reception of the Doppler tone provides an indication that the needle is aimed at the vessel or organ containing the fluid; if the needle becomes misdirected, no backscatter waves are returned and the Doppler tone ceases.
Recognizing the inherent limitations of the highly directional Omizo technique, the '539 patent discloses a system with an omnidirectional transducer located at the needle tip. When used in conjunction with an imaging transducer, the omnidirectional transducer is able to exchange ultrasonic waves with the imaging transducer irrespective of its orientation with the imaging transducer, thereby enabling the '539 system to continually provide a visual marker in the ultrasonic image which indicates the needle tip location. However, the '539 system places several critical demands on its user, such as a physician performing a biopsy procedure. An ultrasonic imaging transducer scans over a relatively planar portion of the body, which is depicted in a two-dimensional image. The needle, however, is free to move in three dimensions as it penetrates the body. Hence, the '539 system operates well for its intended purpose when the needle tip is located within the scan plane, but its operation can be ambiguous when the physician first penetrates the body and attempts to achieve that orientation. Under these initial conditions of the procedure the physician must focus his attention on the insertion and guidance of the biopsy needle as it penetrates the patient's body. At the same time the physician must manipulate the imaging transducer and watch the imaging monitor to simultaneously orient the transducer and needle so that both the tissue structure which is to be biopsied and the needle tip are in the image plane. The simultaneity of both the biopsy procedure and the imaging procedure impose considerable demands on even highly skilled practitioners.
To varying degrees the systems and techniques disclosed in the above described patents enable the tip of an interventional device such as a needle to be sharply visualized in the ultrasonic image by reason of the presence of the active transducer element in association with the needle, and particularly when it is located at the needle tip. These techniques have two significant drawbacks, however. One is the construction of a highly miniamrized transducer for in vivo use, and the accompanying concerns for patient safety. The second is the need for significant system integration required to synchronize signals to and from the biopsy needle transducer with the signals of the imaging scanhead. While potentially offering the advantages of high needle visibility and precision, therefore, these active and invasive techniques pose significant implementation dilemmas.
The principles of a technique for passively visualizing a biopsy needle in a color ultrasound imaging system was reported in the Journal of Ultrasound in Medicine, Vol. 9, at pp 243-45 (1990). There it was noted that the passage of biopsy needles or their guide wires was distinctly evident on color Doppler images as the needle or guide wire was being moved. Hand manipulation of a biopsy needle or guide wire, it was found, provided a color image that corresponded to the shaft of the needle. Such a technique is inherent in the physical principles of Doppler imaging, and is in many cases preferable to the above active techniques by reason of its simplicity and lack of need for additional system integration. The technique suffers shortcomings in that the image of the needle is only highly defined when the needle is being manipulated, and is a coarse representation of the entire needle shaft. Recognizing the aforementioned shortcomings, U.S. Pat. No. 5,095,910, entitled ULTRASONIC IMAGING OF BIOPSY NEEDLE and issued to Powers on Mar. 17, 1992 describes a system for imaging an interventional device with ultrasound. In the ' 910 patent, the tip of a biopsy needle is reciprocated to produce a highly directional motion, thereby causing a Doppler response detectable by a color ultrasonic imaging system. The '910 patent describes reciprocation of a biopsy needle over a wide range of frequencies whereby the reciprocation of the needle tip results in a Doppler shift which is detected via Doppler signal interrogation.
In a copending application Ser. No. 08/022,112 filed on Feb. 25, 1993 and entitled APPARATUS AND METHOD FOR LOCATING AN INTERVENTIONAL MEDICAL DEVICE WITH AN ULTRA SOUND COLOR IMAGING SYSTEM, one of the applicants for the present invention describe a system in which the tip of a needle or other interventional device is visualized in vivo using a color ultrasonic imaging system. Disclosed therein is an apparatus and method for causing a periodic or oscillating mechanical motion in the interventional medical device which results in a significant Doppler shift effect that enables the device to be detected by the color ultrasonic imaging system.
As indicated above, both the '910 system and the system disclosed in U.S. patent application Ser. No. 08/022,112 rely upon the use of a moving needle and a color imaging system to detect the Doppler response produced thereby. While each of these systems avoids the use of invasive transducers and permits the tip of the biopsy needle or other interventional device to be continuously visualized, there are certain drawbacks associated therewith. First, insofar as color imaging systems are designed to be sensitive to the relatively small motion (measurable in microns) of blood in arteries, strong echoes are suppressed and weak echoes enhanced (this is because blood produces weak echoes and tissue artifacts produce strong echoes). Since needles and other metallic interventional devices also produce strong echoes, the device echoes produced thereby are likewise suppressed. Further, the availability to the typical practitioner of the color ultrasound imaging systems contemplated by the '910 patent and the aforementioned application is frequently limited by the large capital investment required for color ultrasound.
It is therefore an object of the present invention to provide an ultrasonic imaging system which does not require the capital investment associated with sophisticated color ultrasonic imaging systems but which still enables the practitioner to continually visualize the needle without the need for the invasive transducers associated with conventional techniques.